Demos: 1N-10 Elastic & Inelastic Collisions

Two identical carts, A and B, are made to collide both elastically and inelastically in this demonstration. Cart A starts from rest on the incline and collides elastically (by means of a stiff spring attached to Cart B) with Cart B, which is initially at rest. Bs speed is determined by measuring the time elapsed as the cart travels a distance s between two photogates. The speed of Cart A just prior to the collision is determined using the conservation of mechanical energy, i.e.

In the inelastic collision, Cart B is reversed and the carts will now stick together because of the Velcro fasteners. When Cart A collides with Cart B, the two stick together and move off with a common speed. Ideally, this speed will be one-half that of the speed found in the elastic collision. (To avoid calculating the speed each time, the times can be compared since the distance is constant. So half the speed means twice the time.)

Directions: Make certain that the spring on Cart B is facing Cart A for the elastic collision. Carefully place the Cart A at the mark on the ramp, be sure the timer is zeroed, and release the cart. The Pasco timer display (which is transmitted through the video camera to the screen) is in the pulse on-pulse-off mode, meaning that when the flag on the cart passes through the first photogate, the timer is triggered on. When it passes through the second gate, it is turned off.

Suggestions for Presentation: That the Cart B takes on the speed of Cart A after the collision is a point that should be developed prior to the demo. You might remind the students that the first collision result can be determined only by combining the conservation of momentum AND the conservation of mechanical energy, whereas the inelastic result needs only the conservation of momentum.

Applications: Could be applied to collisions of real objects. Also, the same result can be shown qualitatively in Demonstration 1N-2.